Saturday, June 29, 2013

In my last post I echoed one reader’s suggestion that
bridges might be extremely beneficialin rolling out a PRT system. After all, a bridge for PRT could carry the
same number of passengers as much more expensive traditional bridges and could
be routed more flexibly on either side. By
solving these particularly vexing transit problems, PRT could offer cities a
much more compelling value proposition than would otherwise be the case with a
limited starter system. Like an aircraft
that needs to surpass a certain speed to achieve lift-off, PRT systems need a
certain number a stations and a certain amount of track to make enough economic
sense to be worthwhile. These numbers,
unfortunately, are high enough that there are currently almost no interested
parties. A bridge or two might tip the
balance.

This raises the obvious question of just how much
cheaper a PRT bridge could be. One of the first tricks to getting the most span
for the money is to use cable instead of trusses, as in suspension bridges.

Here is a transportation system where they have taken the
concept a bit far, at least for my tastes. (It’s called Aerobus, and this was a temporary
installation, though it did carry quite a few passengers in its short
life...) Note that this is a two-way
(double) track, so that the “M” supports are actually a pair of “A” supports. Also note the arching of the track in the far
section. Presumably weight of the
vehicle is pulling on the main support cables, which are then lifting adjacent
sections. The bottom pic shows what is,
apparently, a switched, off-line station. The one thing I like about that top picture,
though, is that it points out something about suspension bridges (not to be
confused with cable-stayed ones) that is worth pondering. I am referring to the thinness of the track itself
between the vertical “hanger” cables. Since
these cables act as simple skyhooks and are closely spaced, the track needs
very little structure to stiffen it. In
this way a suspension bridge can be extremely long, yet the “roadway” portions
each only need sufficient girth to support themselves between hangers… at least in theory! (There
are wind loads and natural resonance issues to factor in.)

Long bridges for lighter loads than for automotive use are
actually quite common. This is because
pipelines (and the occasional ore conveyor) also must also cross natural
obstacles. Here is a pipeline bridge in
China, for example.

Notice
the cabling on the sides to prevent movement from crosswinds. This is actually a fairly standard feature in
pipeline bridges. Here is another example,
although I have to say that I find the parallel main cable design puzzling for
a single pipe. I like the bottom one,
which splays those support cables to handle cross-winds.

Since
bridges for transit are almost certain to be bidirectional, and since I have
never really explored double track for PRT, I built a couple of 3D models. Note that there is no triangulated truss
structure, since such short spans do not require it. Such sections would simply bolt together
end-to-end, with each spanning the vertical support cables.

While working on the section above, I realized that a
pedestrian walkway would be a simple addition, and so I stopped working on that
model and started playing around with adding precast decking sections.

While working on that, however, I got yet another thought,
which is the matter of emergency evacuation. I guess it doesn’t pay to start modeling
without the full vision of what you really want.

Even with vehicles that are sufficiently
autonomous to render system-wide failures impossible, if a bridge becomes
impassible for any reason there is the prospect of backing up and turning away
a lot of traffic. Depending on the span,
number close stations, time of day, etc. this all might take some time. If the bridge is equipped with U-turns at each
end, however, and traffic is instantly diverted away from even approaching
those, it seems like a bridge could be emptied fairly quickly by backing up,
taking the U, and going back toward the station of origin. There is also the matter of evacuating the
occupants of a hypothetical vehicle which is blocking the way and cannot, for
some reason, even be pushed. And let’s
not forget about ordinary maintenance. This
brings up another thought that I had long ago, which is to have some way to
roll a small hanging platform along the tracks without blocking them. Another thought is simply adding an extra
lane. With crossovers, a middle lane
could be used for diverting traffic for any reason. Wider GRT vehicles would be able to fit on
such a bridge as well. Maybe I’d better
ponder this all a bit more before starting yet another 3D model!

6 comments:

I think this part of PRT planning is quite straight forward since these bridge designs are quite independent of other PRT design decisions. You could use almost similar bridges even for supported and suspended PRT tracks.

The key point is of course the cheap prize tag. It would be useful to have some realistic estimates on how much a PRT bridge of certain length or certain capacity would cost, compared to traditional highway bridges. There are numerous places around the world where people and politicians would like to have bridges, and wonder if their bridge can ever be built or not. For them a cheap (and credible) price tag could be a very interesting proposal. Maybe someone could even build a single (elevator like) PRT bridge that simply takes people from one end of the bridge to the other. It would be easy to expand that proposal to a slightly wider PRT system.

Some more thoughts:- the third lane is useful for many needs, e.g. to carry rush hour traffic in one direction- one can have bridges with PRT + pipelines + electricity- one could build PRT bridges over congested areas- it may be possible to change the track structure of an early bridge (=> early technology decisions will not limit possibilities in the future)

Juho, I wrote a lengthy reply and thought I posted it... I guess not. In summery, yes, 3 lanes seems compelling, and that's a good point about utilities. Whereas the idea is not new, it's got a new twist in this instance. I wonder how it would be worth to the electric company, for instance, to get such a crossing. I guess it would eliminate a bunch of transformers, if nothing else...

Bruffie, actually there are a couple of lines that were supposed to indicate a bike lane in the bottom picture. Pretty narrow for anything like serious bike traffic.

Thanks Andrew! You've given me a good idea for a post... This is a matter that goes way beyond bridges...

In this specific instance I would point out that the direction of the middle lane could switch back and forth pretty quickly without humans to get confused. Also, the lane could be used for empties only, with the added benefit of a emergency or maintenance access. It does, though, raise the question of where "pods" go, at night, to roost!

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I would love to hear from you. I might even include your thoughts in a future post. (Only with your permission of course)Just email me- danverhoeve@gmail.com.

Want to download editable copies of the designs pictured in this blog? These (generally done in Google’s free 3D program“SketchUp”) are now available at our design collaboration site,http://code.google.com/p/openprt/Instructions for sharing, improving and updating designs via SVN are also posted there, under “Downloads.”

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ABOUT THIS BLOG

Everyone knows that automobiles, as we know them, cannot continue to be the primary way the world commutes forever. There is the emergence of an enormous commuting class in the developing world, but it’s not just that there will not be enough fuel. The cost will be so high as to humble the world’s great powers, and be the cause of future wars. The impact on Global warming will be catastrophic. The health effects for residents of large cities is already significant. The lost productivity caused by grid-locked traffic currently takes a big bite out of our standard of living that we don’t even recognize. It will only get worse. Clearly something needs to be done.The problem is that there is, currently, no alternative. The car now offers the only real transportation system which has a prayer of offering a traveler a nearly straight line, nearly non-stop, essentially door-to-door journey. (I am leaving out bicycles and motorcycles, and scooters and Segways, etc. because they can all be problematic in bad weather, for older persons, etc.) Enter the PRT, or PERSONAL RAPID TRANSIT. . Though promising in theory, this concept has failed to deliver, and I think I know why. The problem is that the PRT systems developed thus far were developed not to solve this great world problem, but rather to create a business. This bias toward profit has poisoned the decision making process and created products with limited usefulness. I am not against profit. It is just that there has been too little emphasis on what a city wants and too much on what the corporation wants to offer. The system needs to be designed as a versatile, practical set of solutions, not as a product offering that reflects the strength and weaknesses of the company offering it. Proprietary or patented specifications and single source suppliers can only discourage a city government from taking the leap of faith needed to make PRT a reality.Enter this blog. This is but a timid first step on what could be a world-changing journey. What if the complete specifications for a working PRT system were developed and continually improved via posting online? What if the system was modular, with all of the various parts and pieces being individually ready to be out-sourced to the lowest bidder? What if the next company interested in being a PRT provider didn’t have to re-invent the wheel? What if the next city government considering PRT could have easy access to the thoughts and experiences of their peers in other cities? What if all the concerns - environmental, macro- and micro-economic, political, technical, fiscal, legal, even psychological- could be so completely vetted that cities world-wide could institute PRT programs with confidence and competence? So get on board, spread the word…and let’s change the world!